(1) Field of the Invention
The present invention relates to a rotator driving device for rotationally driving a rotator, such as a photosensitive drum provided in a copier, color printer, facsimile, or the like, and also relates to an image forming apparatus using the rotator driving device and a method of rotating such a rotator.
(2) Related Art
In general, a high degree of uniformity is required to rotate a rotator. The reason why the high-degree uniformity is required is given for a case of a photosensitive drum provided as a rotator in, for example, a copier.
The photosensitive drum is rotated in one direction, and a laser beam scans the surface of the photosensitive drum in the direction of the axis of rotation (i.e. the main scanning direction) at every scanning cycle. As a result of scanning, an electrostatic latent image is formed on the surface of the photosensitive drum. If the rotational speed of the drum is unstable, that is, if the drum has nonuniformity in rotation, nonuniformity accordingly occurs in the distances between the scanning lines. This causes inconsistency in the print density on a reproduced image and so deteriorates the image quality. For this reason, a high level of uniformity is required in the rotational speed of the photosensitive drum.
The nonuniformity in rotation is caused by various factors. It may be caused due to eccentricity of the photosensitive drum and thus occur in a cycle of one rotation of the drum. The nonuniform rotation at a low frequency may be caused by torsion of a motor shaft and a load shaft. Meanwhile, the nonuniform rotation at a high frequency may be caused by improper engagement of gearwheels or a timing belt.
To raise the level of uniformity in rotation, a reduction device having a planetary roller instead of gearwheels or a timing belt has been used. By means of this reduction device, the nonuniformity in rotation at a high frequency can be eliminated. This technique is disclosed in Japanese Laid-Open Patent Application Nos. 5-53381 and 5-180290, for example.
The nonuniformity in rotation at a high frequency that is caused by the improper engagement of the gearwheels used as a reduction device can be improved by a rotator driving device that has a planetary roller. However, there is a possibility that the nonuniformity in rotation at a low frequency would increase due to factors, such as a skid of the planetary roller. This problem occurs not only to a photosensitive drum provided in an image forming apparatus. It commonly occurs to other kinds of rotator driving devices that each have a reduction device with a planetary roller and that require uniformity in the rotational speed.
Additionally, a rotator driving device conventionally has to be provided for each rotator, such as a photosensitive drum or developing roller. Therefore, these rotator driving devices to be equipped in an apparatus, such as a copier, occupy a large space, thereby making hard to manufacture the apparatus as compact as possible.
To realize a compact rotator driving device, Japanese Laid-Open Patent Application No. 4-245261, for instance, discloses a technique of rotationally driving a plurality of rotators using only one driving source. To be more specific about this technique, a servomotor that has a reduction device with a planetary roller directly drives one of the provided rotators, and then a rotational force of the servomotor is sequentially transferred to the other rotators via idler rollers.
In this case, however, attention should be paid to that the rotators are mechanically connected to each other via the idler rollers, meaning that the neighboring rotator and idler roller are in contact with each other. With this construction, if low frequency elements, such as nonuniformity in rotation or vibration, occur to one of the rotators, these elements may be transferred to the other rotators via the idler rollers. In addition, if such low frequency elements occur to one of the idler rollers, the following rotators would be adversely affected by this increased nonuniformity. Thus, the aim of realizing a driving device that has a high degree of precision cannot be achieved.